1. Field of the Invention
The present invention relates to image forming apparatuses that use electrophotographic techniques. Here, image forming apparatus refers to an apparatus that forms an image on a recording medium using electrophotographic image forming. For example, electrophotographic copiers, electrophotographic printers (for example, laser beam printers, LED printers and the like), facsimile machines, and word processors and the like are included.
2. Description of the Related Art
Conventionally, a structure such as that shown in FIG. 2 is known in image forming apparatuses that use an electrophotographic process for forming a full color image constituted by a plurality of toner images. A developing unit 8 (8Y, 8M, 8C, and 8Bk), a photosensitive member 5 (5Y, 5M, 5C, and 5Bk), which is a first image carrier, and a process unit that acts on the photosensitive member for each color are set as an image forming unit (hereinafter, referred to as “image forming station”) for each color. There are image forming apparatuses having configurations in which these stations are arranged in a line in opposition to a second image carrier (also referred to as an “intermediate transfer member”) 12, then toner images of each color are superimposed and transferred onto the second image carrier 12, where they are then transferred together onto a recording material 2 by a secondary transfer unit 18. This method is widely used at present since it is possible to have a large number of sheets on which an image is formed per unit of time when forming color images.
The image forming station has at least a process cartridge 22 (22Y, 22M, 22C, and 22Bk). Furthermore, a toner cartridge 11 (11Y, 11M, 11C, and 11Bk) and a toner supply unit 23 (23Y, 23M, 23C, and 23Bk) are provided to supply toner to the image forming station. Then, a configuration is employed that uses a motor as a supply unit drive source (drive unit) for carrying out a rotational action of an agitator member that agitates the toner inside the process cartridge and an action of supplying toner from the toner cartridge to the process cartridge. Here, the process cartridge 22 (22Y, 22M, 22C, and 22Bk) is constituted by the first image carrier 5 (5Y, 5M, 5C, and 5Bk), a charging unit 7 (7Y, 7M, 7C, and 7Bk) that charges the image carrier, and the developing unit 8 (8Y, 8M, 8C, and 8Bk) that supplies toner as a developer to the image carrier. The toner accommodated in the toner cartridge 11 (11Y, 11M, 11C, and 11Bk) as a developer is supplied to the process cartridge by the toner supply unit 23 (23Y, 23M, 23C, and 23Bk).
In regard to the cartridges used in image forming apparatuses, there is a tendency to increase the capacity of toner accommodated in the cartridges since it is common for large quantities of sheets to be printed out in keeping with the increased speeds of image forming apparatuses. And in cartridges having a large toner capacity, there is a tendency for a phenomenon (hereinafter referred to as “packing”) to occur of the toner compacting such that the density of the accommodated toner increases approximately 30 to 40%. Occurrences of this packing phenomenon are caused by such factors as long periods of nonuse and vibration of the cartridge while in a same posture during transportation. When this packing phenomenon occurs there is an increased rotational load on the agitator member that agitates the toner and the toner supply unit 23, and depending on the state of the packing, rotation of the agitator member may become difficult and for example it is possible that the agitator member itself will become damaged. Furthermore, it is also conceivable that the motor, which is the drive system, will lose synchronization. (Here “density” signifies an amount of toner per unit of volume.)
As a solution to this problem, an apparatus has been proposed (Japanese Patent Laid-Open No. H01-297677) in which an agitator member is rotated slowly by setting a rotation velocity of a motor at a velocity lower than an ordinary rotation velocity at a time of initial rotation of the motor, which rotationally drives an agitator that agitates toner. By causing the agitator member to rotate slowly there is no sudden application of load even when the packing phenomenon is occurring. Accordingly, packing can be alleviated without damaging the agitator member or causing the motor to lose synchronization.
However, problems such as the following arise when the initial rotation velocity is slower than the ordinary rotation velocity. Namely, it is necessary to provide a separate control circuit for carrying out a special action of causing the motor to rotate at a speed different from the time of initial rotation or to provide a speed-varying mechanism at the drive transmission mechanism between the motor and the agitator member. Adding configurations such as these is a cause of cost increases for the entire apparatus.
On the other hand, there is increased noise accompanying greater speeds in image forming apparatuses, such as driving noises when driving the apparatus and vibration noises produced by the charging member or the like vibrating due to the voltage (AC voltage component) applied to a conductive roller, which is a charging member. That is, a problem arises when speeds are increased in image forming apparatuses in that there are increased noises harsh to the ear of the user. In particular, since the user is near the printer at times such as when power to the image forming apparatus is turned on or when a cartridge is being loaded, the user may feel discomfort in regard to the noises produced at those times.
As described above, along with increased speeds in image forming apparatuses, that is, increased processing speeds, there is increased torque for the motor that drives the image forming stations or the like in the image forming apparatus, and along with increased torque there is a tendency for noise produced during operation to increase.
Techniques such as the following are known for addressing this problem. In Japanese Patent Laid-Open No. 2004-118028, an initial operation such as when loading a process cartridge is operated at a second processing speed slower than a processing speed of an image forming mode in which an image is formed on a commonly used paper such as plain paper. That is, detection is carried out for a signal indicating that power to the image forming apparatus has been turned on, a signal indicating that a process cartridge has been loaded, or a signal corresponding to a level of compaction of developer that has been filled into a process cartridge. Then, when any of these signals is detected, selection is preformed such that the initial operation of the process cartridge is carried out at the second processing speed. That is, the load placed on the driving motor is reduced at times when there is a high probability the user is near the apparatus, for example, when turning on the power or when loading a process cartridge. In this way, noises such as the driving noise of the motor and the above-mentioned vibration noises can be reduced.
Furthermore, in Japanese Patent Laid-Open No. 2002-369588, a configuration is employed to prevent the motor from losing synchronization by switching the drive current of a stepping motor in response to load fluctuation. That is, the drive current for a stepping motor is switched in response to the stepping motor that drives some of the mechanical elements of the image forming apparatus and load produced when driving such some mechanical elements.
In this regard, in order to achieve device compactness and reduced costs in the color image forming apparatuses, a configuration is sometimes employed in which a motor is shared among a plurality of image forming stations as a drive source for the toner agitator members that agitate the toner and the toner supply units. In this case, use of a single motor is achieved through a configuration capable of selectively enabling or disabling transmission of drive between the motor, which is the drive source, and the various components, thus employing a configuration in which operation of the plurality of toner agitator members and toner supply units is performed selectively with the single motor.
However, in configurations where a motor is shared in this manner, there is a tendency for the causes of fluctuation and the range of fluctuation in the load on the motor to further widen due to such factors as the number of toner agitator members and toner supply units being operated simultaneously and the condition of the cartridges. That is, drive objects to be driven by the single motor increase and therefore the causes of load fluctuation increase and the range of load fluctuation becomes very large. Accordingly, it is necessary to have a motor that has a capacity to withstand high loads and a motor that has a capacity to tolerate fluctuation in the rotation velocity and fluctuations in the drive current. However, a motor such as this is high in cost and therefore there is a problem in that device cost reductions are hindered.